Selective Catalytic Reduction (SCR) technology has been broadly used in reducing NOx emissions of internal combustion engines, especially diesel engines. In a SCR system, typically ammonia (NH3) needs to be mixed with exhaust gas of an engine and then the result mixture passes through a catalyst where ammonia reacts with NOx in the exhaust gas and reduces NOx to nitrogen and water. Due to safety concerns and difficulties in transportation and storage, in SCR systems, normally ammonia is generated from a precursor, such as urea, rather than being used directly. The precursor is also called reductant.
Both solid and liquid reductants can be used in a SCR system. Generating ammonia from solid reductants, e.g. metal ammine salts, such as magnesium ammine chloride (Mg(NH3)6Cl2) and calcium ammine chloride (Ca(NH3)8Cl2), and ammonium salts, such as ammonium carbamate (NH4COONH2) and ammonium carbonate ((NH4)2CO3), has a few advantages compared to dosing liquid urea solution (e.g. DEF or Diesel Exhaust Fluid), including no freezing temperature, no deposit concerns in the decomposition pipe, higher density and lower volume, insensitivity to impurities in the reductant, and no extra energy needed for heating water in the urea solution. However a hindrance for using solid reductants is the issues in delivering the reductant, including high energy consumption, pressure variation, and delivery rate control problems. These issues make it difficult to deliver solid reductants accurately as required.
Normally, to use solid reductant in generating ammonia, reductant in an air-tight container is heated as taught in [Chemical Engineering Science 61 (2006) 2618-2625], and then ammonia gas is released to exhaust air after a pressure is built in the container. Since when heating the solid reductant, all reductant in the container is heated, high heating power is needed and it is difficult to control the pressure in the container, especially when the quantity of reductant is large, due to time delay caused by heat transfer. Changes in pressure affect ammonia delivery accuracy, especially when a feedback control, which may significantly increase system complexity and cost, is not available. And overly high pressure may also create safety concerns.
To solve the problems mentioned above, it is then an objective of the present invention to provide an ammonia generating and delivery apparatus in which an average ammonia releasing rate can be controlled by controlling not only the temperature of a reductant, but also the releasing time, thereby average heating power can be lowered and a more precise control of ammonia releasing rate can be achieved.
A further objective of the present invention is to provide a closed loop pressure control in the ammonia generating and delivery apparatus for obtaining a stable ammonia pressure.
Yet another objective of the present invention is to provide an ammonia delivery control in the ammonia generating and delivery apparatus controlling ammonia delivery rate with a feedback loop including only virtual sensors, so that an accurate ammonia delivery rate can be obtained without significantly increasing the system complexity and cost.
Yet another objective of the present invention is to provide an ammonia generating and delivery apparatus with short response time, so that ammonia can be quickly delivered.